Film digitization can be a low-cost and practical alternative to digital radiographic approaches such as storage phosphor computed radiography (CR) or direct digital image capture (CCD). In the absence of digital image capture, hybrid imaging systems can provide an expedient and low cost solution for both dentists and insurance companies. Without the more significant capital investment required for direct digital devices, the dental practitioner can have the advantages that the digital modality can offer, such as imaging capability to practice management, larger format imagery, and electronic insurance claims. Insurance companies are also motivated to advance to the electronic domain in order to reduce the overhead associated with manual claims submission. U.S. Pat. Nos. 5,164,993 and 5,633,511 generally disclose the construction of tone-scale look-up-tables.
An important step in any hybrid imaging system is the initial image capture, in this case the scanning process. Exposure conditions should be carefully controlled in order to capture the image more optimally so that further degradation of the image in subsequent steps can be minimized. In practice this means controlling unwanted exposure or flare and maintaining proper alignment of the scanned images with the optical configuration. There is much variability among dental radiographs. Both varying x-ray exposure and processing of the radiograph contribute to this. This means that the dental x-ray can range from a very light "under-exposed" image to a very dark in appearance "over-exposed" image. Where there is too much exposure on the platen for an under-exposed radiograph, for example, the captured image will not have enough gray value content. Another important consideration during image capture is the alignment of the radiograph on the platen in the correct horizontal and vertical directions to maintain consistency with the CCD array of the digitizing scanner. If the image is not aligned properly during scanning, image rotation may be required, which can alter the aspect ratio of the original image. To minimize the effects of unwanted flare and image rotation, it is desirable that a template be used that will surround a small film with a dark mask. Alternatively, images can be in a holder typical of the dental practitioner's record keeping.
There is much in the literature that provides approaches for rendering digitally captured imagery. What has not been found is image processing that is specific to dental radiographs or addressing the problem of image capture using a flat-bed scanner and transparency adapter where the initial image capture can be degradated by excessive flare, and that incorporates specific information about the nature of the dental radiograph into an image processing algorithn.
U.S. Pat. No. 5,345,513 discloses the method for enhancement is based on histogram analysis of the digitized x-ray film image so that each pixel represents a density value. In particular, this analysis is demonstrated on chest x-rays The histogram has unique regional signatures, each corresponding to anatomical structures, such as heart or lung regions. The image is then processed dynamically by constructing a piecewise contrast curve specific to chest x-ray anatomy. This method relies on the characterization of the histogram to provide region specific information for rendering the image, but does not address the problem of digitizing film.
U.S. Pat. No. 5,283,736 addresses the need for medical diagnosis from a digitized radiographic image providing a means for signal dependent image processing by thresholding relevant image areas to determine key values for each desired image area The image data from desired regions of interest are used to determine a preferred density range, or a tone scale map.
U.S. Pat. 5,172,419 provides a system to correct exposures of x-ray film thus bypassing the need for second exposure so that an image acquisition, storage and display device can be used to improve image density. This system includes a film densitometer for digitizing film, storage, image processing and film recorder. The digitizer produces a representation of the optical densities for each pixel. Reference data (LUT) is stored consisting of optical densities and corresponding exposure for radiographic films. Using the LUT corresponding to the desired film type, the digitized data is compared to the table to determine the exposure of the image and a correction value is calculating and used to modify the pixel data so that the transformed pixels can be mapped for hard copy output (film) yielding the enhanced x-ray image European Patent Application 90200969.5, entitled "Processing and Digitizing Apparatus for Medical Radiographic Film," provides the means for processing and digitizing exposed film, offering a way to automatically digitize conventionally produce radiographs so that patient information is associated with each image for archival purposes.
U.S. Pat. No. 5,483,325 describes an accessory frame that can be mounted on a photocopier or electronic flatbed scanner that facilitates location and removal of small opaque originals, such as business cards. This patent does not address the problem of flare in copying transparent originals.
Where the processing of radiographs is controlled, a sensitometric curve can be relied upon to adjust pixel values found in the corresponding digitized radiograph. This technique requires more precise knowledge of the exposure levels, film and sensitometry, usually found in larger medical facilities. In the dental community that this work addresses, the wide range in variability of film type used and in film processing would make this technique unfeasible for digitized dental films.